Food casing of nonderivatized cellulose

ABSTRACT

A cellulose food casing comprising a nonderivatized cellulose film precipitated in tubular form from an extruded thermoplastic solution of cellulose, an amine oxide cellulose solvent and water.

This application is a continuation in part of U.S. Ser. No. 08/135,647 filed Oct. 14, 1993 which is a division of prior U.S. application Ser. No. 07/822,506 filed Jan. 17, 1992 now U.S. Pat. No. 5,277,857.

TECHNICAL FIELD

The present invention relates to cellulose based products and more particularly to a tubular cellulose food casing.

BACKGROUND OF THE INVENTION

Cellulose food casings are well known in the art and are widely used in the production of stuffed food products such as sausages and the like. Cellulose food casings generally are seamless tubes formed of a regenerated cellulose and contain a plasticizer such as water and/or a polyol such as glycerine. Plasticization is necessary because otherwise the cellulose tube is too brittle for handling and commercial use.

Cellulose food casings generally are used in one of two forms. In one form the casing consists of a tubular film of pure regenerated cellulose having a wall thickness ranging from about 0.025 mm to about 0.076 mm and made in tube diameters of about 14.5 mm to 203.2 mm. The second form is a reinforced casing wherein the tubular wall of the casing consists of a regenerated cellulose bonded to a paper web. Such reinforced casings are commonly called "fibrous" casings to distinguish them from the nonreinforced cellulose casings. Fibrous casings have a wall in the range of 0.050 mm to 0.102 mm thick and are made in diameters of about 40.6 mm to 193 mm or greater.

The cellulose for making both types of casings is most commonly produced by the so called "viscose process" wherein viscose, a soluble cellulose derivative, is extruded as a tubular film through an annular die into coagulating and regenerating baths to produce a tube of regenerated cellulose. This tube is subsequently washed, plasticized with glycerine or other polyol, and dried. Drying usually is accomplished while the tube is inflated with air at a pressure sufficient both to maintain a constant tube diameter and to orient the film.

The viscose process for making cellulose is well known in the art. Briefly, in the viscose process a natural cellulose such as wood pulp or cotton linters first is treated with a caustic solution to activate the cellulose to permit derivatization and extract certain alkali soluble fractions from the natural cellulose. The resulting alkali cellulose is shredded, aged and treated with carbon disulfide to form cellulose xanthate which is a cellulose derivative. The cellulose xanthate is dissolved in a weak caustic solution. The resulting solution or "viscose" is ripened, filtered, deaerated and extruded.

For food casing the viscose is extruded as a tube through an annular die and about a self centering mandrel into coagulation and regenerating baths containing salts and sulfuric acid. In the acidic baths the cellulose xanthate, e.g. viscose, is converted back to cellulose. In this respect, the acid bath decomposes the cellulose xanthate in a chemical reaction with the result that a pure form of cellulose is coagulated and regenerated. Initially, the coagulated and regenerated cellulose is in a gel state. In this gel state the cellulose tube first is run through a series of rinse water dip tanks to remove by-products formed during regeneration. The gel tube then is treated with a glycerine humectant and dried to about 10% moisture based on total casing weight. As noted above, the gel tube is inflated during the drying process to a pressure sufficient to provide a degree of orientation to the dried cellulose tube.

Both nonreinforced cellulose casings and fibrous casings are produced in this fashion except in the case of fibrous casings the viscose is extruded onto a tube of paper prior to entering the coagulation and regenerating baths.

During regeneration of the cellulose from the xanthate solution, sulfur products are liberated and gases such as hydrogen sulfide, carbon disulfide and carbon dioxide are released through both the inner and outer surfaces of the gel tube. It should be appreciated that the gases produced as by-products during regeneration are noxious and toxic so their containment and recovery imposes a considerable burden on the manufacturing process. Moreover, gases generated at the internal surface of the extruded tube can accumulate within the tubular casing. Since the tubular casing while in its gel state is expansible, the pressure build up of gases accumulating within the gel casing causes undesirable diameter variations. To prevent this, the gel casing is punctured periodically to vent the accumulated gases. This puncturing process, involving procedures to puncture, vent, and then seal the punctured gel tube, results in an undesirable interruption of the manufacturing process. Also, gases which evolve within the casing wall may become entrapped causing bubbles which weaken the casing and detract from its stuffability.

Also, the casing in its gel state to some extent retains low residual levels of the sulfur compounds produced during regeneration. While care is taken to remove all residual sulfur compounds by washing the gel tube prior to drying, the dried casing may still contain trace amounts of these compounds.

Despite the well-known problems inherent with the viscose process, it nevertheless remains today as the most commonly used process for the production of cellulose casing for the food processing industry.

An alternate cellulose production method involves forming a cellulose solution by means of a simple dissolution rather than requiring prior derivatization to form a soluble substance (as in the viscose process). U.S. Pat. No. 2,179,181 discloses the dissolution of natural cellulose by a tertiary amine oxide to produce solutions of relatively low solids content, for example 7 to 10% by weight cellulose dissolved in 93 to 90% by weight of the tertiary amine oxide. The cellulose in the resulting solution is nonderivatized prior to dissolution. U.S. Pat. No. 3,447,939 discloses use of N-methyl-morpholine-N-oxide (NMMO) as the cyclic amine oxide solvent wherein the resulting solutions, while having a low solids content, nevertheless can be used in chemical reactions involving the dissolved compound, or to precipitate the cellulose to form a film or filament.

More recent patents such as U.S. Pat. Nos. 4,145,532 and 4,426,288 improve upon the teachings of the '939 Patent. U.S. Pat. No. 4,145,532 discloses a process for making a solution of cellulose in a tertiary amine oxide such as NMMO which contains 10-35% by weight of cellulose. This higher solids content, achieved in part by including an amount of water (from 1.4% to about 29% by weight) in the tertiary amine oxide solvent, provides a solution adapted for shaping into a cellulosic article such as by extrusion or spinning. In U.S. Pat. No. 4,426,288 the NMMO-cellulose solution contains an additive which reduces decomposition of the cellulose polymer chain so that molding or spinning substances are obtained with only slight discoloration and which will yield molded shapes distinguished by improved strengths upon precipitation in a nonsolvent such as water.

Using NMMO as a solvent for cellulose eliminates the need for derivatizing the cellulose, as in the viscose process. Consequently it eliminates the disadvantages attendant to the viscose process such as the problems associated with the generation of toxic and noxious products such as gases and sulfur compounds during the chemical reaction which regenerates the cellulose.

However, while nonderivatized cellulose resulting from the process of dissolving cellulose in NMMO eliminates certain problems associated with the viscose process, to applicant's knowledge, NMMO-cellulose solutions heretofore have not been used in the manufacture of cellulose food casings. This perhaps is due in part to the fact that the nonderivatized cellulose solution is thermoplastic with a melting point of about 65° C. so it is normally solid at the temperature heretofore used in the extrusion of viscose (e.g. cellulose xanthate) for producing cellulose food casings.

Also, the nonderivatized cellulose solution at 65° C. has a viscosity significantly higher than the viscosity of the derivatized cellulose heretofore used in the production of cellulose food casings. In particular, nonderivatized cellulose in solution may have a molecular weight of about 80,000 to 150,000 and a viscosity at shear rates normally experienced with a food casing extrusion nozzle of about 60,000 to 100,000 centipoises. The high molecular weight and viscosity is because the dissolution of the cellulose does not affect the degree of polymerization. Viscose for casing manufacture (wherein the degree of polymerization is affected by derivatization) has a molecular weight in the range of about 80,000 to 100,000 for fibrous casing and about 95,000 to 115,000 for nonfibrous casing and a viscosity of 5,000 to 30,000 centipoises.

Accordingly, it is a principle object of the present invention to provide a cellulosic food casing article of a nonderivatized cellulose as opposed to a derivatized cellulose.

SUMMARY OF THE INVENTION

It now has been unexpectedly found that nonderivatized cellulose solutions are suitable for use in making both cellulose and fibrous food casings. Nonderivatized cellulose in a molten state can be extruded as a tubular film into a nonsolvent liquid such as a water bath. For purposes of this specification "nonderivatized" cellulose means a cellulose which has not been subjected to covalent bonding with a solvent or reagent but which has been dissolved by association with a solvent or reagent through Van der Waals forces such as hydrogen bonding. "Nonsolvent" means a liquid which is not a cellulose solvent.

In the water bath, the NMMO solvent is extracted from the extruded solution. This allows a precipitation of the dissolved cellulose to a gel state. The resulting gel state tube can be treated with water, a polyhydric alcohol such as glycerine, or other water soluble softening agent such as a polyalkylene oxide or a polyalkylene glycol prior to drying. As described further herein below, the resulting nonderivatized tubular food casing has a more ordered molecular structure than a comparable food casing of derivatized cellulose. This is manifest by the x-ray diffraction pattern of the casing. Moreover, it has been found that this pattern remains substantially unchanged even after mechanical orientation of the tubular casing. This is not the case with derivatized cellulose wherein mechanical orientation does alter the x-ray diffraction pattern.

The nonderivatized food casing of the present invention also has a more homogeneously dense wall structure than its counterpart of a derivatized cellulose. In particular, microscopic examination shows that, in cross section, the wall of a derivatized cellulose food casing has a more porous or spongy central region than the nonderivatized cellulose food casing of the present invention. A possible reason for this is that the gases and other by-products released in the chemical reaction during regeneration of the cellulose from its xantate solution interferes with the formation of a uniform wall structure.

Based on an evaluation of food casings extruded using the nonderivatized cellulose solution, it appears that such casings have a water permeability at least comparable to casings of derivatized cellulose from the viscose process. Water permeability is important for food casings since the processing of food stuffed casings often involves a cooking or curing process which requires that moisture and flavoring components pass freely through the casing material. Moreover, based on an evaluation of casings, it appears that nonderivatized cellulose casings are stronger than casings made with cellulose derived from the viscose process. In this regard, gel state tubes formed of nonderivatized cellulose can be expanded or inflated for drying to a greater degree than comparable gel state tubes of derivatized cellulose as is described in more detail in U.S. Ser. No. 08/434,709 filed May 4, 1995. A possible reason for this is that the more homogeneously dense wall structure of the nonderivitized cellulose gel tube can be drawn and thinned to a greater degree than the more porous wall structure of the derivitized cellulose gel tube. In any event, improving moisture permeability while maintaining or improving strength is unexpected since previous attempts to increase the water permeability of the casing made with derivatized cellulose resulted in a lowering of casing strength.

Casings made of a nonderivatized cellulose also appear to have a comparable, if not greater resistance to tearing when wet than its viscose cellulose counterpart. Wet tear resistance is an important property for food casings as it is a measure of the ability of the casing to withstand abuse imparted by the rigors of shirring and stuffing. For example, it is not uncommon for poorly maintained stuffing equipment to nick or otherwise damage the casing during the stuffing process with the result that the casing tears and fails under the stuffing pressures. Consequently, it is important that the casing be able to receive some amount of damage without failing.

The surprising advantages of a cellulose food casing which has a high degree of order to its molecular structure with out mechanical orientation, which has a unique x-ray diffraction pattern that is not substantially altered by mechanical orientation, has improved wet tear resistance with no loss of strength or water permeability, which is sulfur free, and which is formed by a process that does not generate toxic and noxious by-products, are all attainable through the use of a nonderivatized cellulose to form the food casing.

In particular, the present invention is a cellulose food casing comprising a seamless extruded tubular film of a nonderivatized cellulose precipitated from a thermoplastic cyclic amine oxide solution and said tubular film containing a water soluble softener.

The invention also is characterized by a tubular cellulosic food casing article having chains of cellulose molecules arranged in a nonrandom fashion such that the chains have order both in the direction of the chains and in a plane normal to the chain direction. This degree of order is manifest by an x-ray diffraction pattern of the tubular film which has equatorial spots at spacings of about 8.6 Å and 17.5 Å, four meridional arcs at spacings of from about 11.7 Å to about 2.6 Å and a diffuse ring having its equatorial span greater than its meridional span.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing showing an arrangement of apparatus for extruding the food casing of the present invention;

FIG. 2 is a view of an extrusion nozzle partly broken away and in section as used in the apparatus of FIG. 1;

FIG. 3 is a view similar to FIG. 2 showing an alternate extrusion nozzle.

DETAILED DESCRIPTION OF THE INVENTION

Several tests were conducted to establish the feasibility of using nonderivatized cellulose to form cellulose food casings and to determine the properties of food casings so formed.

For purposes of testing, applicant obtained a NMMO-cellulose solution from Courtaulds Research Ltd., Coventry England. The test solution or "dope" is a yellow/brown solid having a melting point of 60°-70° C. The Material Safety Data Sheet for the dope identifies it as containing, by weight, 70-80% NMMO, 10-20% cellulose and 5-15% water. It is believed the dope as supplied is made according to processes described in U.S. Pat. Nos. 4,145,532, 4,196,282 and 4,255,300 the disclosures of which are incorporated herein by reference.

The dope, received as slabs, was put into a freezer to increase its brittleness. It was then hammered into smaller pieces and ground to a particle size of less than about 1.58 mm in diameter. These particles were then loaded into a Brabendar Model 100 screw-type extruder feeding a die assembly arranged to extrude a tube approximately 25.4 mm in diameter. The extruder and die temperatures were held at about 105° C. The extruded dope was passed sequentially through a screen pack profile of 60/200/325/200/60 mesh screens located between the extruder and die. The screen pack prevents partly melted dope and other contaminants from being extruded and increased the residence time of the dope in the extruder. This insures that the dope is thoroughly melted prior to extrusion.

The die is positioned to extrude a tube downwardly into a water bath held at a temperature of about 15° C. Between the die and water surface was an air gap of about 50 mm. With this arrangement the extruder was operated at about 50 RPM at a feed rate of about 1.13 to 1.36 kg/hr. The residence time of the extruded tube in the water bath was about one (1) minute and from the bath the tube was drawn through nip rolls and wound on a spindle.

FIG. 1 illustrates the test apparatus used in a laboratory pilot operation. In this respect the extruder 12 includes a hopper 14 into which the pulverized dope is fed. The screen pack 16 is between the extruder and the die 18. Band heaters 20 adjacent the die maintain the die at about 105° C. as noted above. The die is oriented so the extruded tubular food casing 22 passes vertically down into water bath 24. As extrusion begins the tube is held open so it fills with water as it enters the bath and is laced around rollers 26. Water within the tube prevents the tube from blocking (i.e. self-adhering) as it is flattened in its passage around rollers 26.

The die 18 is shown in cross section in FIG. 2. The die includes an outer member 28 having a central bore 30. Centered within this bore is a plug 32. The plug and bore surface define an annular space 34 therebetween wherein the space at the outlet 38 defines a gap about 0.508 mm wide and 25.4 mm in diameter. Molten dope introduced into this space through inlet 36 flows down and around the plug and is extruded as a tube from outlet 38.

Tubes extruded in the manner described above were washed in water to remove all NMMO and then samples were soaked in a 10% glycerine solution for ten minutes. The tubes were then inflated with air and dried wherein the pressure of the inflating air was sufficient to maintain the tube open during drying. The wall thickness of the tubes extruded in this experimental operation were not as uniform as the wall thickness of commercially extruded viscose from a conventional casing manufacture. For example, in one casing sample formed of the nonderivatized cellulose dope, the wall thickness at points about its circumference ranged between 0.06 to 0.11 mm whereas a conventional derivatized viscose cellulose casing is a relatively uniform 0.06 mm thick. In any event, this test established that the derivatized dope could be extruded in tubes having a wall thickness down to at least 0.06 mm.

WATER PERMEABILITY

To analyze the water permeability characteristics, a length of the nonderivatized cellulose tube from the laboratory pilot plant operation was soaked in water (25° C.) for about five minutes and thereafter discs 38 mm in diameter were cut from the soaked length. The soaked disc was inserted into an Ultrafiltration Cell as sold by Amicon Corporation (Model 52) having a polyethylene plate to support the test sample. Water at 25° C. under pressure of 500 mm Hg was introduced into one side of the cell. A pipette attached to the other side of the cell collected the water passing through the casing sample. The data collected was then used to calculate the water permeability of the test sample. The resulting calculation represents a water permeation or ultrafiltration rate per unit area per unit of applied pressure, the specific units being ml/min/m² /mmHg. Three samples of the nonderivatized cellulose were analyzed in this fashion. For purposes of comparison, a section of food casing of derivatized cellulose made according to the viscose process was obtained from a commercial production line for Viskase Corporation's NOJAX® casings while still in a gel state (after regeneration, rinsing and treating with glycerine but prior to drying). This gel casing was inflated with air to maintain it open during drying and thereafter it was subjected to the same permeability test. The calculation results are shown in Table I.

                  TABLE I                                                          ______________________________________                                         Ultrafiltration (ml/min/m.sup.2 /mmHg)                                         SAMPLE     NONDERIVATIZED                                                                               DERIVATIZED                                           ______________________________________                                         1          2.53          1.02                                                  2          1.33          1.00                                                  3          1.19          0.94                                                  Average    1.68          0.98                                                  ______________________________________                                    

The analyses results of Table I indicate that the water permeability of casing tested which was formed of the nonderivatized cellulose is greater than that of the derivatized samples.

STRETCH BURST

The strength of the nonderivatized cellulose casing was analyzed by a so called "stretch burst" test. This test involves soaking a section of the casing in room temperature water for thirty minutes to insure it is in a rewet condition. In its fewer condition one end of the sample is tied off and the sample is inflated with air at a rate of pressure increase of about 44.52 mmHg/sec until it bursts. The diameter increase during inflation and the diameter at burst are recorded. The sample of casing formed from nonderivatized cellulose tested had a rewet flat width of 49.5 mm (31.5 mm diameter). In the test it attained a diameter of about 48.0 mm and burst at an internal pressure of 203 mmHg.

For a burst pressure comparison, a length of derivatized cellulose food casing in a gel state was removed from a commercial production line after treating with glycerine but prior to drying. This gel state casing was intended for production of a conventional frankfurter casing known as Viskase Corporation's size 25N NOJAX® casing. This gel state casing was treated in the same manner as the nonderivatized casing noted above, i.e. inflated with air to facilitate drying with no attempt to orient it by inflating to impart a stretch to the casing. The casing then was rewet in the same manner as the nonderivatized cellulose casing sample and subjected to the stretch burst test. In its rewet condition the flat width of three samples averaged 35.47 mm (22.6 mm diameter). The average burst pressure was 271.33 mmHg and the average diameter at burst was 37.33 mm. The burst pressure was higher than that of the nonderivatized sample. However, all factors being equal, burst pressure is determined in part by casing diameter in that the smaller diameter casing will have the higher burst pressure. Accordingly, it is believed that this increase, at least in part, was due to the fact that the average diameter of the derivatized cellulose sample was smaller than the nonderivatized cellulose sample.

As a further attempt to compare burst pressures, two oriented casings of the Viskase Corporation formed of derivatized cellulose were selected for comparison of the rewet burst pressures. One known as Viskase size 40G NOJAX® casing was selected because it is specified as having a flat width (48.77 to 52.83 mm) comparable to the sample of nonderivatized cellulose. Viskase specifications for this size 40G casing includes a minimum burst pressure of 173 mmHg wherein burst pressure is determined according to the same procedure noted above. The second casing known as Viskase size HS 65 casing has a greater specified flat width of (55.37 to 65.02 mm) and the Viskase specifications for this size HS 65 casing includes a burst minimum pressure of 171 mmHg. These minimum values are lower than the burst pressure of 203 mmHg obtained when testing the casing of nonderivatized cellulose.

It also is known that a casing formed of derivatized cellulose has a higher burst pressure after orientation (by drying while inflated to stretch the casing) than without such orientation. The casings formed of nonderivatized cellulose used in the stretch burst test were not oriented whereas the specified burst pressure for the Viskase casings noted above are for oriented casings. Accordingly, it is believed that if oriented, the casings formed of the nonderivatized cellulose will have still higher burst pressures than comparable casings formed of derivatized cellulose. In view of the tests and comparisons made as noted above, it is believed that a casing formed of a nonderivatized cellulose has a burst pressure higher than the specified minimum burst pressure of the closest comparable casings formed of derivatized cellulose.

TEAR STRENGTH

Tear strength in both the machine (MD) and transverse (TD) directions of casing formed of nonderivatized cellulose also was tested. Samples to be tested were cut as rectangles from both nonderivatized cellulose and size 25 N NOJAX® gel state casing extruded and treated as noted above. One group of three samples was taken such that one side (A) of each sample is cut perpendicular to the MD direction. A second group of three samples was taken such that one side (B) is cut perpendicular to the TD direction. A slit for initiating the tear was made in each sample. Each slit started at the middle of one side (A) or (B) and extended in the direction to be tested. The samples were soaked in demineralized water for thirty (30) minutes, measured for thickness and then fixed to a DuPont Pendulum-type tear tester. The sample is arranged so that when the pendulum of the instrument is released, the weight of the swinging pendulum will tear the sample in half wherein the tear is initiated by the slit cut into one side. A scale of the instrument records a value representing the tear force in gram-centimeters required to tear the sample. This value is used to calculate the tearing force per millimeter of film thickness.

Casings made of both derivatized and nonderivatized samples were tested using the same test procedures. The results of the tear test on samples formed of the derivatized cellulose are shown in Table II. Table III shows the results of tear tests on samples cut from tubular film formed of nonderivatized cellulose.

                  TABLE II                                                         ______________________________________                                         Tear Strength                                                                  (Derivatized Cellulose)                                                        ______________________________________                                               Sample         MD Tear    gm-cm                                                Thickness (mm) Force (gm-cm)                                                                             per 0.1 m                                      ______________________________________                                         1.    0.063          10.5       16.6                                           2.    0.063          10.9       17.3                                           3.    0.065          12.5       19.2                                           4.    0.066          13.2       20.0                                                        Average MD  18.28                                                              Tear Force/0.1 mm                                                 ______________________________________                                               Sample         TD Tear    gm-cm                                                Thickness (mm) Force (gm-cm)                                                                             per 0.1 m                                      ______________________________________                                         1.    0.065          18.0       27.6                                           2.    0.066          21.4       32.4                                           3.    0.066          19.4       29.3                                           4.    0.066          19.6       29.7                                                        Average TD  29.75                                                              Tear Force/0.1 mm                                                 ______________________________________                                    

                  TABLE III                                                        ______________________________________                                         Tear Strength                                                                  (Nonderivatized Cellulose)                                                     ______________________________________                                               Sample         MD Tear    gm-cm                                                Thickness (mm) Force (gm-cm)                                                                             per 0.1 mm                                     ______________________________________                                         1.    0.098          42.9       43.7                                           2.    0.124          66.5       53.6                                           3.    0.149          69.2       46.4                                                        Average TD Tear                                                                           47.9                                                                Tear Force/0.1 mm                                                 ______________________________________                                               Sample         MD Tear    gm-cm                                                Thickness (mm) Force (gm-cm)                                                                             per 0.1 mm                                     ______________________________________                                         1.    0.120          35.7       29.8                                           2.    0.154          73         47.4                                           3.    0.160          67.3       42.1                                           4.    0.169          103.4      61.2                                                        Average TD 45.1                                                                Tear Force/0.1 mm                                                 ______________________________________                                    

A comparison of the results show that on average, the tear strength per 0.1 mm of thickness is significantly greater in both the MD and TD directions for the tubular film formed of nonderivatized cellulose. Particularly, the MD tear strength on average is over 2.5 times greater and the TD tear strength is 1.5 times greater. This is important because as discussed above, wet tear strength is an indication of the ability of the casing to withstand the rigors of stuffing. Tear strength also is important during casing converting operations prior to stuffing. For example food casings such as those used to process frankfurters and the like typically are converted by a shirring operation (i.e. longitudinally compressing into closely packed pleats) so as to gather a long length of casing into shorter tubular sticks. It is not uncommon for a shirring operation to compact upwards of sixty (60) meters of casing to a length of only 76 cm and the shirring process itself is conducted at speeds of up to 360 meters per minute. Accordingly, it is important that the casing be able to maintain physical integrity and not tear during the shirring operation. Thus for both processing (shirring) and stuffing considerations, it is desirable to have a casing with high tear strength.

Stuffing tests were conducted to determine if casings made of a nonderivatized cellulose were suitable for food processing. Several pieces of the casing, formed, treated with glycerine and dried as noted above, were tied at one end and hand stuffed with a high collagen frankfurter emulsion. Some casings split on stuffing due to weak spots caused by pinholes or bubbles in the casing wall. Those samples which were successfully stuffed were processed after stuffing by cooking at 82° C. and 40% RH for 75 minutes to simulate conditions in a commercial operation. Casings of nonderivatized cellulose, stuffed and processed in this fashion, did not split or otherwise rupture during cooking, indicating a suitability for use as food casings.

The die used to produce casing for the previous tests was replaced with a nozzle arrangement having some features similar to a nozzle used in the production of cellulosic frankfurter casing from viscose. A typical viscose extrusion nozzle for producing cellulose food casing includes a self centering mandrel which extends out from the body of the nozzle. The nozzle is located below the level of the regenerating liquid and is oriented so the extruded viscose tube passes vertically upward from the nozzle and along the mandrel up and out of the regenerating liquid.

For purposes of extruding the molten nonderivatized dope, modifications as described hereinbelow were made to facilitate the extrusion of the more viscous thermoplastic dope of nonderivatized cellulose and the nozzle was arranged to extrude downwardly into the nonsolvent bath.

FIG. 3 shows the extrusion nozzle to include an outer member 40 having a central bore 42 which tapers towards an outlet 44. Centered within the bore is a tapered plug 46. An annular extrusion passage 48 is defined between the plug and the bore surface wherein the passage outlet 44 is about 0.25 mm wide and about 23.6 in diameter.

The molten dope from the extruder (FIG. 1) enters extrusion passage 48 through an inlet port 50 and then flows around and along the plug to the extrusion passage outlet. The surface of the plug is provided with a spiral channel 51 intended to facilitate the even distribution and passage of the molten dope around and along the plug.

Extending axially from the plug and out of the bore is a hollow mandrel 52. The lowermost end portion 54 of the mandrel, in use, is disposed below the level of the precipitating liquid indicated at 56.

The lowermost end portion 54 has a diameter about equal to the diameter of the extrusion passage outlet 44. However, a length of the mandrel between its lowermost end portion and the extrusion outlet is relieved to provide a seat 57. Retained in this seat is a sleeve 58 formed of a relatively low friction material such as tetrafluroethylene (TFE). The sleeve is tapered from a smaller end adjacent the extrusion outlet 44 to a larger end which matches the diameter of the mandrel end portion 54. The tapered, low friction surface provided by this sleeve facilitates the passage of the extruded tube along the mandrel and into the precipitating liquid.

FIG. 3 further shows that a central conduit 60 extends axially completely through the mandrel and plug. A second conduit 62, disposed concentrically about the first and which extends upward through the plug has a closed lower end 64 and an upper end 66 which is open. This second conduit 62 and the inner wall 68 of the hollow mandrel define an annular channel 70 therebetween. A first port 72 in the wall of the second conduit connects the space 74 between the concentric conduits 60, 62 to the space 70 between the second conduit and the inner surface of the hollow bore. A second port 80 extends through the wall of the mandrel and the sleeve. This second port 80 is located adjacent the smaller sleeve diameter and above the level of the nonsolvent precipitating liquid.

The concentric conduits 60, 62 permit the circulation of the precipitating liquid to the interior of the extruded nonderivatized cellulose. In particular, the precipitating liquid from bath 82 is pumped, by means not shown, into the space 74 between the concentric conduits through its open top 66 as indicated by arrow 84. This liquid flows through port 72 into space 70 and then out of the mandrel through port 80. This liquid, which flows down along the surface of the sleeve 58 and the mandrel lower portion 54, performs several functions. First it lubricates to facilitate the passage of the extruded nonderivatized cellulose tube (represented in FIG. 3 by dashed line 78) down along the mandrel and into the bath 82. It also fills the extruded tube to facilitate precipitation of the cellulose at the inner surface of the extruded tube. The liquid from within the extruded tube is pumped out through the central conduit 60 as indicated by arrow 86 and back to the bath.

Using this extrusion nozzle with the same dope and extrusion conditions as previously noted, a tube of the NMMO-cellulose solution was extruded into a 25° C. bath containing 30% NMMO and 70% water by volume. At this concentration, the combination of water and NMMO is a nonsolvent for the cellulose.

The residence time of the extruded tube in the bath was about one (1) minute. This was sufficient to precipitate the nonderivatized cellulose from the solution. The extruded tube was then washed with water and immersed in a 10% glycerine solution for ten minutes. The tube was then inflated with air and dried to produce casing samples for testing. The casing sample produced in this manner contained 38% glycerine by weight of dry cellulose and had a flat width after rewetting of about 30.7 mm (19.57 mm diameter). This, in conjunction with casings made and tested for stretch burst as noted above, indicates that casings having a rewet diameter of between about 20 mm and 31.5 mm may be made from the nonderivatized cellulose in accordance with the present invention. These casings were then subjected to the same permeability, stretch-burst and tear tests as previously described.

A conventional gel state casing of derivatized cellulose was used as a control. This gel state casing, was taken from the production line of Viskase Corporation for manufacturing its size 25N NOJAX® casing because the specified flat width (26.9 mm) diameter (17.2 mm) of this size casing is close to the flat width and diameter of the casing of nonderivatized cellulose produced with the modified extrusion nozzle. The gel state casing was taken at a stage of production after contacting with glycerine and before drying. At this stage the casing has about 13-14% glycerine. It was then inflated just to hold it open and air dried. The resulting casing of the derivatized cellulose had a rewet flat width of 29.46 mm (18.76 mm diameter) so it was still slightly smaller in diameter than the nonderivatized cellulose sample.

The nonderivatized casing produced with the modified extrusion nozzle was thinner and more uniform than previous samples produced. Table IV shows the thickness of eight rewet samples measured at three points around its diameter.

                  TABLE IV                                                         ______________________________________                                         Casing Wall Thickness (mm)                                                     Sample     1      2          3    Average                                      ______________________________________                                         1          0.060  0.058      0.050                                                                               0.056                                        2          0.104  0.072      0.083                                                                               0.084                                        3          0.058  0.065      0.049                                                                               0.057                                        4          0.078  0.087      0.053                                                                               0.073                                        5          0.056  0.053      0.042                                                                               0.050                                        6          0.056  0.053      0.042                                                                               0.050                                        7          0.067  0.066      0.049                                                                               0.061                                        8          0.039  0.062      0.051                                                                               0.051                                        ______________________________________                                    

Table IV shows casings were produced having a wall thickness on average of as small as 0.050 mm with a wall thickness uniformity better than that produced by the die of FIG. 2. On average, the wall thickness of these eight samples ranged from 0.050 mm to 0.084 mm whereas the samples produced using the die of FIG. 2 had a thickness of between 0.06 to 0.11 mm. The results of water permeability, tear and stretch burst test tests conducted on the samples are shown in Table V.

                  TABLE V                                                          ______________________________________                                                Ultra-                                                                         filtration                                                                             Stretch Burst Tear Strength*                                           (ml/Min/                                                                               Pressure                                                                               dia at burst                                                                             gm per 0.1 mm                                        M.sup.2 /mmHg)                                                                         mmHg    mm        MD    TD                                      ______________________________________                                         nonderivatized                                                                          1.66      145     27.18   15.3  29.9                                  derivatize                                                                              0.72      293     44.45   17.6  34.6                                  ______________________________________                                          *average of 4                                                            

Table V indicates that while water permeability (ultrafiltration) was higher for the casings of nonderivatized cellulose, the stretch burst and tear strengths of the nonderivatized cellulose casings produced with the modified nozzle were lower than the derivatized cellulose counterpart. It is believed that the lower stretch burst and tear strength of the nonderivatized cellulose casing as reported in Table V is attributable to both its higher ultrafiltration value (1.66 vs 0.72), and higher glycerine content (38% vs 13-14%) because increasing both are known to have an adverse affect on casing strength. It further is believed the strength of casing of the nonderivatized cellulose would have been above that of the derivatized cellulose casing samples at equivalent glycerine content.

DRAW DOWN TEST

A draw-down test also was conducted to evaluate the suitability of the nonderivatized cellulose for use in the production of fibrous casing. The draw-down test is used to screen papers and viscose solutions to determine suitability for fibrous casing manufacture. In the test, a section of paper used in forming fibrous casing is placed on a flat plate. The liquid to be tested (usually a viscose solution) is poured onto the paper and the paper is drawn beneath a metering bar which squeezes the liquid to a uniform thickness over the paper surface. The coated paper is stretched over a hoop and immersed in a coagulating and regenerating solution, rinsed and then dipped into a bath containing a glycerine solution. An examination then is made to determine whether the coating liquid has penetrated and bonded with the paper. Both of these characteristics (e.g. penetration and bonding) are essential for successful fibrous casing manufacture. If penetration and bonding are observed, then the materials would be suitable for manufacturing a fibrous casing.

For purposes of testing the nonderivatized cellulose, the NMMO-cellulose dope as described above was melted (105° C.) and diluted with a 78% NMMO aqueous solution to produce a solution containing about 5% by weight of cellulose. A sheet of conventional viscose bonded, long fiber 11 pound per ream weight hemp paper (0.030 mm thick) obtained from C. H. Dexter Co., Windsor Locks, Conn. and used in making commercial fibrous cellulose casing was placed on a flat plate and heated to 105° C. The solution was poured onto the paper and the paper was drawn under a horizontal bar spaced 1.02 mm above the plate so the paper was coated with the solution to a thickness of slightly less than 1 mm. After a few seconds the hoop was immersed into flowing tap water for about 10 minutes to precipitate the cellulose. The hoop then was immersed into a 10% glycerine solution for fifteen minutes and oven dried (105° C.).

Visual inspection of the noncoated side of the paper indicated that there was acceptable penetration of the nonderivatized cellulose through the paper and the cellulose did bond to the paper. As noted above, these characteristics are essential for successful fibrous casing manufacture. So by this draw-down test, the nonderivatized cellulose is suitable for manufacturing a fibrous cellulosic casing.

A second sample formed of a 7.3% derivatized (cellulose xanthate) solution was used in a draw-down test with the same 11 pound per ream weight paper. The bar was set to a height of 0.762 mm. While the coating was thinner, about the same amount of total cellulose was applied to the paper because the cellulose concentration of the solution was greater. The coating on the second sample was coagulated and regenerated by immersion in baths containing salts and sulfuric acid and then it was rinsed with water, treated with glycerine and dried.

Tensile tests were conducted on both samples as well as on a control consisting of the paper treated only with a 10% glycerine solution and dried. The results of tests conducted on 25.4 mm wide samples are reported in Table VI.

                  TABLE VI                                                         ______________________________________                                         Tensile Strength                                                                          Thickness  Load at                                                             After Drying (mm)                                                                         Failure (kg)                                             ______________________________________                                         Control      0.030        2.8                                                  Sample 1*    0.210        5.5                                                  Sample 2**   0.125        7.8                                                  ______________________________________                                          *Paper coated with nonderivatized cellulose                                    **Paper coated with derivatized cellulose (viscose)                      

The results of the test show that the nonderivatized cellulose and the viscose add-on both increased the load bearing ability of the casing over uncoated paper although the increase of the sample coated with nonderivatized cellulose was less. The load at failure for Sample 1 formed of the nonderivatized cellulose was about 70% the load at failure of the derivatized cellulose sample. Despite this lower value it is believed that the penetration and bonding characteristics indicate that a commercially acceptable fibrous casing can be formed of the nonderivatized cellulose because the nonderivatized cellulose improved the strength of the base paper and this would be sufficient to satisfy the strength needs of fibrous casing users.

X-Ray Diffraction Test

As a further test, a tubular film of nonderivatized cellulose according to the present invention, and a conventional tubular food casing of derivatized cellulose were both analyzed by x-ray diffraction. A comparison of the x-ray diffraction patterns disclosed that the nonderivatized cellulose film exhibited a higher degree of orientation than the film of conventional derivatized cellulose made using the viscose process. This higher orientation is manifest by a unique x-ray diffraction pattern which did not change even after mechanical orientation of the nonderivatized film.

To perform such analysis, gel state tubular film of nonderivatized cellulose and a gel state film from the viscose process were dried while inflated just enough to hold them open during drying without stretching them. Drying in this fashion was intended to avoid the effects of a mechanical orientation.

Strips about one centimeter wide were cut along the length of the dried tubular film for 2 to 3 cm and each piece was exposed for 19 hours to Ni-filtered CuK α radiation of wave length 1.541 Å from a micro focus x-ray generator at 40 kV and 6 mA. The diffraction pattern was recorded on a flat photographic film in a camera with sample-to-film distance "D" equal to 4.43 cm. The sample chamber was flushed with dry helium in order to minimize air scattering. By measuring the distance "r" of each spot, arc or ring from the center of the diffraction pattern, its characteristic d-spacing (in angstrom units) was calculated using the Bragg equation

    2d sin θ=λ

where tangent 2θ=r/D. In this equation θ is the Bragg angle. The x-ray diffraction patterns are shown in Table VII. The pattern of Sample 4A represents the film of derivatized cellulose and the pattern of Sample 4B represents the film of nonderivatized cellulose (Sample 4B); both samples being formed without mechanical orientation such as by drying the gel state film while stretching it by inflation. Table VII gives the "d" spacings and relative intensity for each of the various features in the diffraction pattern.

                                      TABLE VII                                    __________________________________________________________________________     Comparison of Observed d-spacings (in Å) In The Diffraction Patterns          Derivatized  Derivatized  Nonderivatized                                                                              Nonderivatized                       Item                                                                              Sample 4A    Oriented     Sample 4B    Oriented                             No.                                                                               Item                                                                               Intensity                                                                           d   Item                                                                               Intensity                                                                           d   Item                                                                               Intensity                                                                           d   Item                                                                               Intensity                                                                           d                           __________________________________________________________________________     1  spot                                                                               weak 17.1                                                                               absent       spot                                                                               medium                                                                              17.1                                                                               spot                                                                               medium                                                                              17.1                        2  spot                                                                               weak 8.6 absent       spot                                                                               medium                                                                              8.6 spot                                                                               medium                                                                              8.6                         3               absent       arc weak 11.7                                                                               arc weak 11.7                        4  arc medium                                                                               5.85                                                                              ring                                                                               weak 5.9 arc medium                                                                               5.85                                                                              arc weak 5.9                         5  ring                                                                               strong                                                                              5.2-4.1                                                                            ring                                                                               strong                                                                              5.1-4.1                                                                            ring                                                                               strong                                                                              5.2-4.1                                                                            ring                                                                               strong                                                                              5.4-4.1                     6  arc medium                                                                               3.25                                                                              ring                                                                               medium                                                                              3.2 arc strong                                                                               3.25                                                                              arc medium                                                                              3.2                         7               ring                                                                               very weak                                                                           2.7 arc weak 2.8 arc weak 2.7-2.6                     __________________________________________________________________________

Both x-ray diffraction patterns are characteristics of a sample having orientation both along the polymer chain direction and in a plane normal to the chain direction. This can be inferred from the presence of diffraction along the meridian (north-south through the pattern) and along the equator (east-west through the pattern). The patterns show two weak spots 1 and 2 on the equator. Equatorial spot 1 has a d-spacing of 17.1 Å which denotes lateral separation between adjacent polymer chains having their molecular axes somewhat aligned parallel to the meridian. This alignment of the axes parallel to the meridian is inferred form the meridional arc 4 at a spacing of 5.85 Å. Spot 2 at 8.6 Å falls in place with the second order of spot 1 and clearly is diagnostic of short range lateral organization of the films. The strong and diffuse band 5 which spans a d-spacing of 5.2 to 4.1 Å along the equator is narrower along the meridian. This is an indication that orientation along the meridian is better than along the equator.

What distinguishes the pattern of the nonderivatized cellulose Sample 4B form the derivatized cellulose sample 4A is the presence of meridional arcs 3 and 7 of weak intensity at 11.7 Å and 2.8 Å respectively. These are indicative of a better orientation along the molecular axis of the sample as well as a better lateral organization of its polymer chains.

A further distinguishing feature noted from Table VII is that for the pattern features common to both samples (see items 1, 2 and 4-6) those of the nonderivatized Sample 4B have an intensity equal to or greater than those of Sample 4A. This is shown, for example, by comparing the weak intensity spots 1 and 2 and the medium intensity arc 6 of the derivatized sample with the corresponding medium intensity spots 1 and 2 and strong intensity arc 6 of the nonderivatized sample.

Drying the gel state casing as set out above under minimal inflation pressure preserved the orientation of the as-formed gel state film. Thus, the x-ray diffraction patterns of these films demonstrate that the as-formed nonderivatized cellulose film of the present invention is identifiable by an x-ray diffraction pattern which reveals an unexpected higher ordering of cellulose molecules of the as-formed film than its as-formed derivatized cellulose counterpart.

A further tubular nonderivatized cellulose film was prepared substantially as disclosed herein except that the length of the air gap between the die orifice and the surface of the nonsolvent bath was increased to about nine inches in accordance with the teachings of copending application Ser. No. 08/179,418 filed Jan. 10, 1994 the disclosure of which is incorporated herein by reference. However, while in the gel state, the tubular film was stretched by inflation to about 1.25 its gel state diameter and dried. Drying while holding the tube in a stretched inflated condition served to impart a degree of mechanical orientation to the tube.

A sample cut from the dried tube was subjected to the same x-ray diffraction analysis as Samples 4A and 4B. The features of the resulting x-ray diffraction pattern are reported in Table VII so a comparison can be made between the mechanically oriented nonderivatized film and its as-formed counterpart (Sample 4B).

As shown in Table VII, the oriented sample of nonderivatized casing, even after a mechanical orientation, retains the features of the x-ray diffraction pattern of its as-formed counterpart. The items, intensity of the items and location of the items do not substantially change.

A derivatized tubular film formed by similar treatment (gel state casing inflated and dried at 1.25 transverse orientation) also is shown in Table VII for a comparison with its as-formed counterpart (Sample 4A). In contrast to the oriented nonderivatized sample, the oriented derivatized sample did show a change in the x-ray pattern in terms of both the items seen and the intensity of such items. Items 1 and 2 of the nonoriented Sample 4A were not seen after orientation, the medium intensity arcs 3 and 6 of the nonoriented sample appear as rings in the oriented sample and an additional feature, the very weak ring of item 7, was seen in the oriented sample but not in the nonoriented sample.

The results of x-ray diffusion analysis reported in Table VII demonstrate that the nonderivatized tubular cellulose film of the present invention has a higher degree of ordering than its derivatized counterpart and the resulting pattern can serve to distinguish it from a comparable film of derivatized cellulose.

The higher order, in both the axial and transverse direction, of the nonderivatized tubular film of the present invention, possibly can be attributed to a transition of the molten solution to a liquid crystal or mesophase as the solution is drawn through the air gap. The relatively high concentration of cellulose in the solution (15%), the change in the relative water/NMMO concentration due to flashing off of water from the hot surface of the extruded tube and the resulting cooling of the extruded solution in the air gap all contribute to the transformation to a liquid crystal state. This transition imparts crystallinity and order to the chains of cellulose molecules which is preserved during the subsequent extraction of the solvent and precipitation of the cellulose.

In contrast, the derivatized cellulose (cellulose xanthate or "viscose") includes the xanthate moiety which is pendant from the hydroxyl groups of the cellulose. The pendant xanthate groups may interfere with the tendency to undergo a similar transition to a liquid crystal or mesophase. Also, the relatively low concentration of cellulose (6%) in the viscose system prevents the transition. Consequently, during regeneration by chemical action, the xanthate group is removed and the cellulose increases in density but this process does not appear to impart to the regenerated cellulose film, the degree of order and crystallinity which occurs during the mesophasic transition in the NMMO/water/cellulose system.

It should be appreciated that a food casing article of nonderivatized cellulose represents an advance in the art. An improved and characterizing feature of the cellulose casing of the present invention is that it has a molecular structure more ordered than its derivatized cellulose casing counterpart as manifest by the x-ray diffraction pattern of the casing. Also, its tubular wall structure is more homogeneously dense since it is formed by solvent extraction rather than by a chemical reaction which generates gaseous by-products. Since no sulfur or sulfur compound is used or generated in forming the casing as described herein, the resulting casing is sulfur free. The wet tear strength, water permeability and burst pressure of the nonderivatized cellulose casing of the present invention are at least equal to, if not superior to, a conventional food casing formed of a derivatized cellulose such as is formed by regenerating the cellulose from a cellulose xanthate solution. Since there are no toxic or noxious by-products generated by the manufacture of the casing, the manufacturing process imposes no burden on the environment and represents an advance in the art of cellulose casing manufacture. 

Having described the invention in detail, what is claimed as new is:
 1. A tubular cellulose film comprising a food casing which is a shirred seamless tubular film of nonderivatized cellulose precipitated from an extruded thermoplastic cellulose solution composed of cellulose, a tertiary amine oxide and water, said tubular film having a diameter of at least 14.5 mm containing a water soluble softener and said tubular film having chains of cellulose molecules arranged in a nonrandom fashion such that said chains have order both in the direction of said chains and in a plane normal to the chain direction as manifest by an x-ray diffraction pattern for said film, said pattern having equatorial spots at spacings of about 8.6 Å and about 17.1 Å, four meridional arcs at spacings of from about 11.7 Å to about 2.6 Å and a diffuse ring having its equatorial span greater than its meridional span.
 2. A tubular cellulose film as in claim 1 where said four meridional arcs include arcs at about 11.7 Å and 2.7 Å.
 3. A tubular cellulose film as in claim 1 wherein said four meridional arcs are at about 11.7 Å, 5.9 Å, 3.22 Å and 2.7 Å.
 4. A tubular cellulose film as in claim 1 wherein said equatorial spots have an intensity level greater than the intensity level of at least two of said meridional arcs.
 5. A tubular cellulose film as in claim 1 wherein on a scale of intensity of weak, medium and strong, said equatorial spots are of a medium intensity.
 6. A tubular cellulose film as in claim 5 wherein said meridional arcs include arcs at 11.7 Å and about 2.6 Å which have a weak intensity.
 7. A tubular cellulosic film as in claim 1 wherein said seamless tubular film is sulfur free.
 8. A tubular cellulosic film as in claim 1 including a paper web in the wall of said seamless tubular film.
 9. A tubular cellulosic film as in claim 1 wherein said thermoplastic solution comprises pulp cellulose dissolved in N-methyl-morpholine-N-oxide (NMMO).
 10. A tubular cellulosic film as in claim 1 wherein said thermoplastic solution comprises 70-80% by weight NMMO, 10-20% by weight cellulose and 5-15% by weight water.
 11. A tubular cellulosic film as in claim 1 wherein said water soluble softener is selected from the group consisting of polyhydric alcohols, polyalkylene oxides, polyalkylene glycols, water and mixtures thereof.
 12. A tubular cellulosic film as in claim 1 wherein said water soluble softener is glycerine. 